In recent years, considerable efforts have been made by the coatings industry to develop coating formulations containing little or no volatile organic compound (VOC) content. Regulations to limit the amount of VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial solvent-based coatings operations used to coat such products as automotive parts, appliances, general metal products, furniture, and the like. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC content coating compositions must still meet or exceed the performance standards expected from solvent-based compositions.
Alkyd resins are one of the most common binders used for ambient-cure, solvent-based coatings. The resistance properties of traditional solvent-borne alkyd resins are developed via autooxidative crosslinking of the alkyd film. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to give hydroperoxides, which subsequently decompose to generate free radicals, resulting in oxidative crosslinking. This oxidative crosslinking process is commonly accelerated by adding driers, such as, for example, various salts of cobalt, zirconium, calcium, and manganese. However, while alkyd resins have shown, and continue to show, promise, they have relatively slow “dry” and/or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes polymerization of an alkyd resin with a vinyl compound, such as styrene or methyl methacrylate, via a free-radical reaction, to produce a vinyl-alkyd copolymer or a vinyl alkyd. Vinyl alkyd resins generally have a higher molecular weight and a higher Tg, producing coatings with reduced tack-free time (via solvent evaporation). However, the through-dry time (oxidation of the film) of such coatings is longer due to the decreased degree of unsaturation in the alkyd as a result of copolymerization with the vinyl compound. This problem is described in further detail in Resins for Surface Coatings, Vol. 1, pp. 181 et seq., ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, which is incorporated herein by reference. An additional drawback is that paint formulations containing vinyl alkyd resins require greater amounts of solvent, due to the increased molecular weight and Tg of the vinyl alkyd.
JP 48085628 (hereinafter JP '628) describes light-curable coating compositions made from a drying oil-modified alkyd resin which is further modified using glycidyl acrylate, glycidyl methacrylate, or its derivative. In this reference, drying oil-modified alkyd resins having carboxyl groups and an oil length of 20-80 are treated with glycidyl acrylate, glycidyl methacrylate, or its derivative, in the presence of a polymerization inhibitor. In a specific embodiment, a drying oil-modified alkyd resin having an acid number of 100 and an oil length of 34 is reacted with 36 parts glycidyl methacrylate, to give a resin varnish having an acid number of 5.0.
The formulations are made by a process in which hydroquinone, a polymerization inhibitor, is used in the reaction mixture, to prevent the alkyd from gelling during resin synthesis. A drawback of this approach is that the presence of a polymerization inhibitor in paint formulations is known to prolong the drying times of the resulting coating films.
U.S. Pat. Nos. 6,534,598 and 6,548,601 disclose ambient oxidative-cure compositions based on acrylate-functionalized alkyd resins. These ambient oxidative-cure compositions may be used, for example, in fast drying, high solid paint or enamel compositions or formulations. However, we have found these acrylate-functionalized alkyd resins to be less storage-stable than commercial applications demand. Their viscosity and molecular weights tend to increase over time. As a result, paint formulation becomes difficult and the resulting coating properties are inconsistent.
U.S. patent application Ser. Nos. 10/121,927 and 10/121,396, filed Apr. 12, 2002 and presently pending, likewise disclose ambient oxidative-cure compositions that include an acrylate-functionalized alkyd resin. U.S. patent application Ser. No. 10/356,286, filed Jan. 31, 2003 and presently pending, discloses acrylate-functional alkyd coating compositions that exhibit improved dry time.
U.S. Pat. No. 4,571,420 discloses a composition comprising an acrylic polymer backbone having pendant unsaturation, at least about 0.3 equivalent of polyfunctional mercaptans per equivalent of unsaturation, and 0.001 to about 0.15 wt. % vanadium compound. This reference teaches the utilization of about 0.3 to about 5 equivalents of a polyfunctional mercaptan as a crosslinker to react the pendant unsaturated groups in the acrylic polymer upon exposure to air.
U.S. Pat. No. 6,476,183 discloses a coating composition comprising an oxidatively drying polyunsaturated condensation product, such as an alkyd, having pendant groups comprising unsaturated carbon-carbon bonds; a polythiol having two or more —SH groups; and a siccative. This reference teaches the utilization of a polyfunctional thiol as a crosslinker, to improve the oxidative cure of an alkyd, thus requiring a polythiol having two or more —SH groups.
JP 2075669 describes a primer composition prepared by mixing an oxidation-curable alkyd resin, a dryer, a fat or oil, and/or a silane coupling agent. The alkyd resin may be a resin-modified or phenol-modified alkyd resin, the fat or oil may be a condensate of a natural unsaturated fat or oil or an unsaturated higher fatty acid with an alcohol, and the optional silane coupling agent may have an amino, mercapto, or epoxy group.
U.S. Pat. No. 4,016,332 discloses a polymer prepared by reacting a polyester or polyol with a mercaptan comprising a hydrocarbon chain of 1 to about 18 carbon atoms, and substituted with at least one carboxyl group, to form a mercapto-terminated reaction product, and reacting this product with one or more ethylenically unsaturated monomers in the presence of a peroxygen or azo catalyst to form the polymer. The mercapto-terminated reaction product may act as a chain transfer agent in the subsequent reaction with the ethylenically unsaturated monomer. The polyester can be oil-modified, an oil such as sunflower oil being employed in its preparation. The oil-modified polyester preferably contains 20-80 wt. %, more preferably, 30-65 wt. % of the oil component.
U.S. Pat. No. 4,116,903 discloses a storage-stable emulsion that includes an interpolymer containing about 5-90 wt. % of a preformed water-soluble alkyd resin, the balance being at least one in situ emulsion polymerized vinyl monomer. Preferred vinyl monomers include styrene and acrylic and methacrylic esters. The product interpolymer contains, on a 100 wt. % total non-volatile solids basis, preferably 5-90 wt. %, more preferably 15-50 wt. %, typically 25-50 wt. % alkyd resin, the balance being in situ polymerized vinyl monomer. Other materials that can be present at the time of or during emulsion polymerization include chain transfer agents such as n-octyl mercaptan or t-dodecyl mercaptan.
U.S. Pat. No. 3,932,562 discloses a process for producing a resin that comprises graft polymerizing by chain transfer onto a mercapto group-containing alkyd resin one or more ethylenically unsaturated polymerizable compounds in a weight ratio of 5:95 to 70:30, preferably 10:90 to 60:40. The ethylenically unsaturated compounds are selected from a large group of monomers that includes glycidyl carboxylates. The mercapto group-containing alkyd resin can be formed by introducing a mercapto group into the alkyd resin molecule either in the course of or after the preparation of the alkyd resin, but prior to the chain transfer reaction with the one or more ethylenically unsaturated compounds.
U.S. Pat. No. 4,234,466 discloses a process for preparing a solid pigment spherical particle dispersion that comprises: subjecting to suspension or bulk polymerization in the presence of a suspension stabilizer a liquid composition comprising at least one ethylenically unsaturated polymerizable compound selected from a group that includes glycidyl vinyl compounds such as glycidyl methacrylate, at least one resin selected from a group that includes alkyd resins having at least one mercapto group to be grafted with the ethylenically unsaturated compound, and at least one pigment. The polymerization can be carried out in the presence of a chain transfer agent such as a mercaptan, for example, t-dodecyl mercaptan. The ethylenically unsaturated polymerizable compound and the resin containing at least one mercapto group are used in amounts of, respectively, 27-96 wt. % and 3-60 wt. % on the basis of the weight of the liquid composition.
U.S. Pat. No. 4,517,322 discloses a non-gelled resin formed by reacting in an aqueous medium an ethylenically unsaturated polyester having an ethylenically unsaturated component containing two carboxylic acid moieties with ethylenically unsaturated monomers that include an ethylenically unsaturated carboxylic acid. Relative to the total weight of the non-gelled resin, the ethylenically unsaturated polyester and the ethylenically unsaturated monomers are reacted in amounts of, respectively, about 90-30 wt. %, preferably about 80-35 wt. %, and about 10-70 wt. %, preferably about 20-65 wt. %. The ethylenically unsaturated polyester can be an alkyd resin prepared in the presence of a drying, semi-drying, or non-drying oil or fatty acid. The ethylenically unsaturated carboxylic acid can be combined with another monomer such as styrene or an acrylic or methacrylic ester. In illustrative Example 1-B, the reaction of the ethylenically unsaturated polyester and the ethylenically unsaturated monomers is carried out in butyl cellosolve in the presence of the chain transfer agent mercaptoethanol.
U.S. Pat. No. 4,983,716 discloses a process for producing a non-aqueous dispersion of addition polymers in an alkyd medium that comprises: forming an alkyd having a non-volatile content greater than about 75% by reacting a triglyceride oil with a trifunctional carboxylic acid or anhydride, and then reacting the resulting product with a trifunctional alcohol or a mixture of tri- and di-functional alcohols to form the alkyd. One or more monomers are polymerized in the presence of the alkyd by a free radical mechanism. The triglyceride oil can be sunflower oil, and the trifunctional acid or anhydride can be trimellitic acid or anhydride. The monomer is selected from a group of compounds that includes multifunctional epoxides. The polymerization mixture contains alkyd and polymerizable monomers in amounts of, respectively, 25-99 wt. %, preferably 30-60 wt. %, most preferably, 40-55 wt. %, and 1-75 wt. %, preferably 40-70 wt. %, most preferably, 45-60 wt. %. According to the '716 patent, a chain transfer agent such as methyl mercaptopropionate or 2-mercaptoethanol is added to the polymerization mixture in an amount of from about 0.1-6.0 wt. % of the monomer.
U.S. Pat. Nos. 5,340,871 and 5,348,992 contain similar disclosures pertaining to aerosol compositions containing non-aqueous dispersions of addition polymers in alkyd media. The non-aqueous dispersion preferably comprises the reaction product of two or more monomers, at least one of which has hydroxy functionality, and the polymerization is carried out in the presence of a chain transfer agent selected from the group consisting of methyl mercaptopropionate, dodecyl mercaptan, and 2-mercaptoethanol. The polymerization mixture contains alkyd and polymerizable monomers in amounts of, respectively, about 35-75 wt. %, preferably about 40-60 wt. %, and about 65-25 wt. %, preferably about 60-40 wt. %. The alkyds are typically formed from triglyceride oils or fatty acids thereof, for example, sunflower oil. Other starting materials include multifunctional alcohols such as pentaerythritol and multifunctional carboxylic acids or anhydrides such as phthalic acid and trimellitic acid.
U.S. Pat. No. 6,051,633 discloses a process for producing an air-dry alkyd-stabilized non-aqueous dispersion of addition polymers in an alkyd medium. The polymerization is carried out in the alkyd medium via a free radical addition mechanism in the presence of a mercaptan-containing chain transfer agent that preferably is selected from the group consisting of methyl mercaptopropionate, dodecyl mercaptan, and 2-mercaptoethanol. The polymerization mixture contains alkyd and polymerizable monomers in amounts of, respectively, about 25-75 wt. %, preferably about 40-60 wt. %, and about 75-25 wt. %, preferably about 60-40 wt. %. There remains a need in the art for an acrylate-functional alkyd resin that is storage-stable and commercially viable for ambient oxidative-cure, fast-dry coatings having low VOC.